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AD-76 7 2 99

MAGNETIC BUBBLE MATERIALS

Jerry W. Moody, et al

Monsanto Research Corporation

Prepared for:

Advanced Research Projects Agency

II August 1973

DISTRIBUTED BY:

im National Technical Informatien Service U. S. DEPARTMENT OF COMMERCE 5285 Port Royal Road, Springfield Va. 22151

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MRC-SL-386

FINAL REPORT

CD |s«

MAGNETIC BUBBLE MATERIALS

Monsanto Research Corporation St. Louis, Missouri 63166

Jerry W. Moody, Robert M. Sandfort, Roger W. Shaw

Michael C. Wlllson, Joseph T. Cheng, and Richard J. Janowiecki

Contract No. DAAH01-72-C-1098

Distribution of this document is unlimited

ARPA Support Office Research, Development, Engineering,

and Missile Systems Laboratory U.S. Army Missile Command

Redstone Arsenal, Alabama is

\ v

.

Sponsored by:

Advanced Research Projects Agency ARPA Order Nr. 1999

Reproduced by

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3 HEPOUT TITLE

I». REPORT SECURITY CLASSIFICATION

UNCLASSIFIED 2b. GROUP

MAGNETIC BUBBLE MATERIALS

< DESCRIPTIVE NOTES (Type ol report and Inclusive dales)

Final Report, 11 January-11 July 1973 S A'j TMORIS» (First nttme, middle initial, last name) — ——_

J. W. Moody, Robert M. Sandfort, Roger W. Shaw, et al.

6 RtPOW T D A T E

20 September 1973 Bo CONTRACT OH GRANT NO

DAAHO1-72-C-1098 6. PROJEC T NO

ARPA Order No. 1999

7a. TOTAL NQ. OF PAGES

INATOR'S REPORT NUMBER(S)

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MRC-SL-386

ab. OTHER REt : RT NO(SI (Any other numbers that may be assigned in t s nport}

10 OIS TRIBUTION STATEMENT

Distribution of this document unlimited

11 SUPPLEMENTARY NOTES

13 ABSTRACT

12- SPONSORING MILITARY ACTIVITY

Advanced Research Project Agency Washington, D.C.

The Czochralski growth of defect-free, non-magnetic garnet crystals and the preparation of epitaxial magnetic bubble garnet films by liquid phase tpitaxy (LPE) and arc-plasma spraying (APS) were investigated. Two garnet systems, Eu- Yb-Y and Sm-Y, were identified as being of particular practical interest. These garnets exhibit reasonable mobilities, satisfactory quality factors, and good temperature stability. Device-quality films can be deposited by LPE from PbO-Bj O3 solution on (111) GGG substrates. Methods were developed for growing epitaxial garnet films by APS. However, the films were usually contaminated with crystallites of orthoferrite, hematite, magnetoplumbite or other phases. Low-defect-density films were not pre- pared and the elimination of unwanted phases by compositional control appears to be a formidable problem. All recent developments indicate that the LPE "dipping" growth of garnet films is a commercially viable process. Core-free single crystals of GGG with defect densities of less than 5 per cm2 can be grown readily by the Czochralski method. Such crystals, up to 38 mm in diameter, are now available commercially.

DD .Fr„1473 UNCLASSIFIED

K Security Classification

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vttmm&T&***™v!?*??™T*T*™r^^

UNCLASSIFIED Secimty Ctaaaification

'.«V WONOS

Magnetic bubble materials

Magnetic garnets

Gadolinium gallium garnets

Crystal growth

Liquid phase epitaxy-

Chemical vapor deposition

Arc plasma spray-

Substrate preparation

Rare-earth iron garnets

NOLC

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1 UNCLASSIFIED

Security CUasificction

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FINAL REPORT

MAGNETIC BUBBLE MATERIALS

Monsanto Research Corporation St. Louis, Missouri 63166

Jerry W. Moody, Robert M. Sandfort, Roger W. Shaw, Michael C. Willson, Joseph T. Cheng, and Richard J. Janowiecki

August 11, 1973

Contract No. DAAH01-72-C-1098 ■ |

'3 :973-

.- Distribution of this document is unlimited J

ARPA Support Office Research, Development, Engineering

and Missile Systems Laboratory U.S. Army Missile Command Redstone Arsenal, Alabama

Sponsored by: Advanced Research Projects Agency

ARPA Order No. 1999

IC

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SUMMARY

This is the final report on the project "Magnetic Bubble Materials,

Phase II, " Contract No. DAAH01-72-C-1098. It covers in detail the work

performed from January 11, 1973 through ,Tuly 11, 1973. The Czochralski

growth of defect-free non-magnetic garnet crystals , the preparation of

epitaxial magnetic bubble garnet films by liquid phase epitaxy (LPE) and

arc-plasma spraying (APS) were investigated. References to earlier work

are included for the sake of completeness.

Two garnet systems, Eu-Yb-Y and Sm-Y were identified as being

of particular practical interest. These garnets exhibit reasonable mobilities,

satisfactory quality factors and good temperature stability. Device quality

films can be deposited by LPE from PbO-BzC^ solution on (11]) GGG sub-

strates.

Methods were developed for growing epitaxial garnet films by APS.

However, the films were usually contaminated with crystallites of ortho-

ferrite, hematite, magnetoplumbite or other phases. Low defect density

films were not prepared and the elimination of unwanted phases by composi-

tional control appears to be a formidable problem.

All recent developments indicate that the LPE "dipping" growth of

garnet films is a commercially viable process. This method is now used

almost exclusively by workers in the field.

Core-free single crystals of GGG with defect densities of less than

5 per cm2 can be grown readily by the Czochralski method. Such crystals,

up to 38 mm in diameter, are now available from commercial vendors.

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TABLE OF CONTENTS

1.0 INTRODUCTION

2.0 CHARACTERIZATION

2, 1 Film Thickness

2. 2 Defect Detection and Location

2. 3 Characteristic Length and Domain Dimensions

2.4 Saturation Magnetization and Magnetic Fields

2.5 Coercivity

2.6 Anisotropy

2.7 Temperature Effects

3. 0 LIQUID PHASE EPITAXY

3. 1 Review

3.2 Experimental

3.3 The Eu-Yb-Y System

3. 3. 1 Introduction

3. 3. 2 Properties of Eu-Yb-Y Garnet Films

3. 3. 2. 1 Magnetization

3.3.2.2 Characteristic Length

3. 3. 2. 3 Anisotropy

3.3.2.4 Coercivity

3.3.2.5 Mobility

Page

1

3

3

3

3

4

4

4

5

7

7

B

9

9

10

10

12

12

15

15

ii

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3.4 The Sm-Y System 21

3.4.1 Introduction 21

3.4.2 Properties of Sm-Y Garnet Films 23

3.4.2.1 Magnetization 23

3.4.2.2 Characteristic Length 23

3.4.2.3 Coercivity 23

3.4.2.4 Anisotropy 26

3.4.2.5 Mobility 2 7

3.4. 2.6 Temperature Coefficient of Characteristic Length 28

3.5 The Nd-Y System 26

3.6 Discussion 30

4.0 PLASMA SPRAYING 31

4. 1 Introduction 31

4. 1. 1 Growth from the Melt 31

4.1.2 Growth from the Solution 32

4. 2 Experimental 33

4.2.1 R.F. Plasma Spraying Station 33

4.2.2 D. C. Plasma Spraying Station 37

4,2. 3 Mate-ial 37

4. 3 Results and Discussion 39

4. 3. 1 D. C. Plasma Spraying 41

4.3.1.1 Epitaxial Crystal Growth from the Melt 41

4. 3. 1. 2 Epitaxial Crystal Growth from Solution 41

111

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4. 3. 2 R. F. Plasma Spraying

4. 3. 3 Characteristics of Selected Garnet Films

5. 0 BULK CRYSTAL GROWTH

6. 0 CONCLUSIONS AND RECOMMENDATIONS

7.0 REFERENCES

8.0 APPENDIX

8. 1 Growth Parameters for Eu0i5Yb0jl5Y2.35Fe3#8Galt2O12

8. 2 Growth Parameters for Sm0i36Y2.64Fe3.7Ga1.3O12

43

44

47

48

51

54

54

54

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LIST OF FIGURES

Page

Figure 1 Magnetization vs Gallium Content for Eu-Yb-Y Films 11

Figure 2 Characteristic Length vs Gallium Content for Eu-Yb-Y Films 13

Figure 3 Optical Hysteresigraph Tracings of Neel Temperature Determination of Eu-Yb-Y Films 14

Figure 4 Optical Response of a Typical Sample of Eu-Yb-Y Garnet to a Step Rise in Bias Field at Various In-Plane Fields 16

Figure 5 Natural Frequency and Decay Time vs In-Plane Field for a Typical Sample of Eu-Yb-Y Garnet 17

Figure 6 Domain Wall Mobility vs In-Plane Field for a Typical Sample of Eu-Yb-Y Garnet 19

Figure 7 Domain Wall Mobility vs Temperature in a Typical Sample of Eu-Yb-Y Garnet 20

Figure 8 Magnetization vs Gallium Content for Sm-Y Garnets 22

Figure 9 Characteristic Length vs Gallium Content for Sm-Y Garnets 24

Figure 10 Coercivity vs Gallium Content for Sm-Y Garnets 25

Figure 11 Mobility vs In-Plane Field for Some SmyY3_yFe5.xGax012

Garnets f 29

Figure 12 Schematic of R.F. Plasma Spraying Station 34

Figure 13 R.F. Plasma Torch Detail 35

Figure 14 D. C. Plasma Spraying Station 38

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Table 1

Table 2

Table 3

Table 4

Table 5

LIST OF TABLES

Uniaxial Anisotropy of Some Sm-Y Garnets

Powder Compositions

Typical Conditions for Growing Epitaxial Garnet Films from a Flux Using D. C Arc Plasma Spraying

Characterization Data of Selected Bubble Films Made by the Method of R.F. Plasma Spraying

Process Conditions Used for Preparing Selected Representative Films by the Method of R.F. Plasma Spraying

Page

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40

42

45

46

VI

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1.0 INTRODUCTION

Thifj is the final report on the project "Magnetic Bubble Materials -

Ph^se il", Contract No. DAAH01-72-C-1098. This project was a continua-

tion of work performed under Contract No. DAAH01-72-C-0490. The pri-

mary objective of this work was to fulfill a need for a supply of reproducible,

low defect, high quality magnetic bubble materials to support the device

work of various DOD agencies who are investigating the c.pplication of these

materials for end uses of importance to national security. The work has

involved development of methods to grow high quality bulk crystals for use

as substrates, development of substrate polishing and preparation procedures,

studies of epitaxial film growth by three different methods: liquid phase

epitaxy (LPE), chemical vapor deposition (CVD), and arc-plasma spraying

(APS); and development of methods to fully characterize bubble domain

materials.

Two reportsU' ^) have been issued previously which presented in

detail the preparation of garnet films and crystals,and the progress of

bubble materials technology. Two reports' ' 4/ covering characterization

techniques have been issued also. One, '4' a comprehensive, critical survey

of characterization procedures, has been widely accepted as a valuable aid

by workers in the field.

This report covers the work accomplished from. February II, 197 3

through July 11, 1973. Specifically, it discusses the LPE growth of films

of the Eu-Yb-Y and Sm-Y systems and the preparation of garnet films by

APS. In addition, a section is devoted to up-dating characterization

, , ,.,,, ««-««i

procedures. Finally, the report is concluded with a brief survey of bubble

materials technology and recommendations for future work.

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^•0 CHARACTERIZATION

The majority of the methods employed in the characterization of the

garnet films grown under this contract have been dealt with in detail in

earlier reports. (*' 3. 4) The following brief discussion is thus intended

only to summarize these methods and present relevant detail where new

techniques have been instituted since those reports were prepared.

2- 1 Film Thickness

The thickness of the magnetic garnet films has been measured using

optical interference of reflected light(4). Thickness uniformity also has

involved the use of optical interference, in this case in the form of a contour

map caused by interference of monochromatic light reflected from the front

and back of the film. (4)

2. 2 Defect Detection and Location

Defects which impede domain motion are readily observed using a

polarizing microscope and an alternating bias field sufficient to cause most

of the strip domains to move(4). Experience at Monsanto has indicated that

this method, when properly employed, is sufficiently sensitive to detect all

defects likely to cause difficulties in devices of present design.

2. 3 Characteristic Length and Domain Dimensions

Where possible, domain dimensions have been measured on parallel

strip domain patterns in order to improve accuracy. This includes the

determination of characteristic length from the film thickness and strip

domain period. (4. 5. 6) Bubble domain diameters are routinely measured

at the stability limits but such results are of more limited accuracy.

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2.4 Saturation Magnetization and Magnetic Fields

The saturation magnetization was derived from the measurement of

the highest bias field at which normal bubbles are stable (the collapse field)

combined with a knowledge of the zero field strip domain width and the thick-

ness. The theory required has been worked out by Thiele (7), and Fowlis and

CcPeland(5) with useful graphical results in several places. (4) A calibrated

bias coil mounted on the stage of a polarizing microscope served as source

for most of the magnetic fields needed in the work. For situations in which

the sample was inaccessible, as in a variable temperature stage, the satura-

tion magnetization was usually derived from an optical hysteresis loop. (4. 6)

2. 5 Coercivity

A measure of sample coercivity results from determination of the

polarized light intensity modulation resulting from several amplitudes of

bias field modulation. The plot of light modulation vs. field modulation

amplitudes yields an approximately linear relationship which extrapolates,

for zero light modulation, to a finite field amplitude--the coercivity field. (4)

This technique, while a worthwhile relative measure, usually yields values

smaller than the half-width of the complete hysteresis loop and so has little

absolute meaning. Experimentally, samples of coercivity greater than

approximately 0. 5 Oe show measurably degraded characteristics in devices.

2.6 Anisotropy

The measurement of magnetic anisotropy field. HA, by magneto-

optical techniques, was just under development at the time of preparation

of ref. 4 . The recent papers of Shumate, et al. (8' 9) place this measure-

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ment on a much better footing than it was at that time. In particular they

form the basis for a technique proposed by Josephs'^/ and now used almost

exclusively in this laboratory. In this method a large field is applied in the

plane of the film, rotating the magnetization into this plane. An alternating

bias field normal to the film still yields a small optical modulation propor-

tional to the susceptibility in the normal direction. For fields H greater

than HA - 4 TT MS( this susceptibility is given by X = MS/(H + 4TTMS - H^).

Thus a. plot of X"1 (or the proportional inverse light modulation amplitude)

versus H yields a linear relationship with an intercept at X"1 = 0 of

H = HA - 4 TT MS. This linear relationship is quite consistently observed

when the proper alignment of the sample has been established. The

resulting values of H^ are accurate to roughly ± 10%.

It should be noted that the method outlined in ref. 4 (the assignment

of the low field shoulder as H^ - 4rrMs) is in error. This shoulder is, to

within ~10%, et.iial to H^. but its interpretation is not on as sound theoreti-

cal footing as is the method outlined above, so that the latter has been

adopted as standard.

Z. 7 Temperature Effects

The means of measuring the Neel Temperature using a microscope

hot stage and optical detection has been discussed previously. '4' Compensa-

tion temperatures were measured in somewhat similar fashion where,

however, no microscope was used. Instead, light was conducted to th^

sample by means of a fiber optic "light pipe", then polarized, sent through the

sample, analyzed, and brought out to a photomultiplier via another light

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pipe. The sample was in a coil within a Dewar vessel where its temperature

was adjusted by means of location in the Dewar and a heater coil. The coil

provided an alternating bias field and the synchronous optical signal served

as a measure of the state of the sample. At compensation the phase of the

optical signal changes by 180°, generally accompanied by a range of high

coercivity and therefore small or zero signal. A substantial drive field was

applied to keep this range <, 10oC and the compensation temperature was

taken as the center of this range.

Determination of temperature effects on characteristic length,

saturation magnetization, and mobility near room temperature involved the

use of a thermoelectric variable temperature stage in the microscope.

Such an addition made these measurements possible over a range from -10

to +60oC.

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3. 0 LIQUID PHASE EPITAXY

3- 1 Review

Six different film compositions we. e supplied to the Sponsor during

the first phase of this program:

1. Euo.6Er2.4Fe4.4Gao.6Oi2

2. Gdo.8Er2-2Fe4i6Ga04012

3. Gdo.5Y2,5Fe4Ga1012

4. Gd,Yb0 gYj iFe4 2Ga0 8012

5. Gd0-44La0 44Y2 52Fe4GaiO; 2

6. Gdj jTmo gY1Fe4i3Ga0 7Oi2

Although all these compositions were tailored to support stable 5 - 7 mn

bubble domains, they exhibit a wide range and variety of magnetic properties

The anisotropy of compositions 1 and Z is primarily growth induced; that of

composition 3 is stress-induced, while that of compositions 4, 5, and 6 is

probably both growth- and stress-induced. The magnitude of the aniso-

tropies range from about 500 Oe (for composition 3) to 3ö00 Oe (composition

1) while domain mobilities range from less than 100 cm/sec/Oe (composition

1) to over 600 cm/sec/Oe (composition 4).

Because of their range of properties these first compositions were

valuable for basic property studies and preliminary device work. However,

none of the cornpo-iitions possessed the combination of high mobility and

temperature stability that is required for a practical device. Therefore,

during the second phase of the program attention was directed toward

developing compositions with adequate temperature stability which also have

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the desirable features of several of the compositions in tho above list. The

guidelines for compositional development and the requirements for tempera-

ture stability were discussed in detail in the Semi-Annual Technical Report. (2)

Briefly, the discussion pointed out the potentialities inherent in the mixed

Eu-Y. Sm-Y, and Nd-Y magnetic garnets, and these systems were investi-

gated during the second phase of the program. Since Eu exhibits little or no

angular momentum and Eu3Fe5012 has no compensation point, the Eu-Y

system was of particular interest. Magnetic garnet films of the Eu-Y system

were supplied to the Sponsor early in this program. More recently, Sm-Y

garnets were prepared and supplied to the Sponsor. In addition small

additions of Yb were introduced into the Eu-Y garnets to improve domain

wall mobility characteristics and samples of these materials were also

supplied to the Sponsor. The Nd-Y system was also investigated but, in

this case, the magnetization was found to be in the plane of the film.

The preparation and properties of all these magnetic films are

described in this section of the report. Some of this work was presented

previously in the Semi-Annual Technical Report. U) it is repeated here for

the sake of completeness.

3. 2 Experimental

The apparatus and methods used to grow magnetic garnet films by

LPE have been described previously. U. 2) The magnetic bubble films we]

grown by the isothermal dipping method^ 1) from PbO-B^ solutions. The

films were grown both statically and with rotation. However, during this

;re

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phase of the program most films were grown horizontally at a rotatioii rate

of ZOO rpm to ensure thickness uniformity.

The work was directed toward developing solution compositions in

which could be grown magnetic films which supported stable 5-7 |am bubbles.

In practice, a series of films with constant rare earth ratio but with varying

Ga content was prepared by adding appropriate increments of Ga203 to the

solution. The magnetic properties of the films were measured and the films

were examined for evidence of excessive tension (cracking) or compression

(faceting). Those solution compositions which produced sound, smooth films

having the desired magnetic properties were then used to grow films for

delivery to the Sponsor.

All films were deposited on Syton-polished, (111) GGG substrates.

The thickness of the films ranged from about 3 to lOfam. The thickness

uniformity of films grown with axial rotation was about ± 1 percent.

3.3 The Eu-Y-Yb System

3. 3. 1 Ircroducti'jn

The preparation of E^YjFeg.xAlxO^ and Eu0-6Y2#4Fe5_xGaxO12 films

were described in the Semi-Annual Technical Report. '^J The high Eu, Al-

containing films were characterized by high coercivities («0. 5 Oe) and low

domain wall mobilities (<100 cm/sec/Oe in zero in-plane field). The low Eu,

Ga-containing films also exhibited relatively high coercivities («0. 3 Oe) and

domain wall mobilities of about 200 cm/sec/Oe in zero in-plane field. This

value is about the minimum acceptable mobility for magnetic bubble devices.

Bonner, et al. ^ ' found that small additions of Yb would reduce the dynamic

»KJIBilSWllWM'WMWW' -J ■-,'-' •• ■—'-■' -■-■■■■ ■' ■- ■ '■■■■•■■■■

lUIHUUMIIMIIiilii«! «WH».!!.»..™. .iii>iiMM>iuiiiiilMiu.i|ii|injiiiii jaiiiivniiuiiiiii. ii i ■ iiiaj WII uiuwip^a IIIULIIIIP j i. IUIUU iiiiin)$piim!M«WiniR««^««imiM«P9m^m«linH|

instabilities which give rise to velocity saturation at device magnitude drive

fields and concomitant low mobility values in Eu-Y garnet films. In other

work at Monsanto, ^^' the effect of Yb additions to the low Eu, Ga-containing

garnet films had been investigated in detail. Because of wide variability

from sample to sample, the dependence of mobility on Yb content was not

clear-cut; however, a definite improvement was obtained. The results

suggested that a maximum mobility, in zero bias field, of about 600 cm/sec/Oe

was attained with about 0.2 Yb ion molecule. For in-plane fields of

about 40 Oe, a maximum mobility of about 1500 cm/sec/Oe was attained with

about 0. 1 Yb ion/molecule. Therefore, an addition of about 0. 15 ion molecule

appeared to be a desir'ble Yb content. Having decided the Yb content, the

Eu, Y and Ga contents were then adjusted to yield smooth, unerased films

which supported stable 5 - 7 |j,m bubbles. The dependence of magnetic pro-

perties on compositions of these films as wel] as previously prepared Eu-Y

films are presented here.

3. 3. 2 Properties of the Eu-Yb-Y System

3. 3. 2. 1 Magnetization

The dependence of room temperature saturation magnetization on

gallium content is shown in Fig. 1. The magnetization of other Eu-Y composi-

tions (prepared previously) are also shown for comparison. Magnetization

decrei-ses with increasing Ga (or Al) but does not depend strongly on the

concentration ratio of rare-earths.

10

.■■.... -.^.^ :. 1 fJlllflll-l t— ■■ ^lA.-..^.^*.-~^^>J^^-.^.1.^^... «>.:.. .ft.^.^..-.* i^- ^—•'l rill in I I - — ■- ..-Jw...,^ —..-...- ■ ■ -^.^■^■. ..■J.,.. ^ ...-..■,-■

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rviAiM.w»i.w*4Trwrvr^-^-,W'r^^*^^

3. 3, 2. 2 Characteristic Length

Characteristic length increases with Ga content as is shown in Fig. 2.

It can be seen from Figs. \ and 2 that the magnetization of films which support

5 - 7 ^im bubbles is about 150 gauss and x is approximately 1. 2 ion/molecule.

The average temperature coefficient of the characteristic length from

0 to 50oC for Eu0#5Yb(U5Y2i3:;Fe3>8Ga1#2Oi2 films (the composition delivered

to the Sponsor) is -0.47 percent per degree.

3. 3. 2. 3 Anisotropy

The uniaxial anisotropy field of Eu-Yb-Y films delivered to the

Sponsor is about 900 Oe, which results in a quality factor of about 6 for this

material. The anisotropy field is somewhat smaller than that of Yb-free,

Eu-Y films grown previously (1200-1400 Oe).

Measurements of the Neel temperature reveal an interesting feature

of films grown in tension which is believed to be related to the magnetic

anisotropy of these garnets. In this work, the Neel temperature is measured

by use of an optical hysteresigraph. (4) The signal for a "normal" sample is

illustrated in curve 'a' of Fig. 3. The Neel temperature id marked by the

sharp drop of the signal to zero as the wall susceptibility drops to zero at TN.

For films grown in tension, the curves display an anamolous "tail" or "peak"

as is shown in curves 'b' and 'c', respectively, of Fig. 3. In these cases, the

Neel temperature is taken as the point at which the signal finally reaches zero.

Since this effect is found only in Eu-containing films grown in tension, it is

believed to be related to a component of anisotropy, probably stress-induced,

which is not perpendicular to the plane of the film. Such a stress-induced

12

mm*. v-^>-^^"'^^--^---—■^"■'■-^^^ ...,-.=.i.-,....^.,>,.^.>Jj.,^-^...^..^.-.....^...u^A, .^^J^..^,^_.,Y..J ....^

■..w,,..,,..,,,, ,. ...J.-I.»^... i i < mi upavmwM^'wi'iu-m'« H ~"^m^^Mm^«nWmnaMnit«i i^mmi™*m*m*

1.0

0.8

Eu2 Y1 Fe5-x Alx 012

a Eu

E

B 0.6

to

a> ■^ o «3

o

0.4-

0-4 Y26 Fe5.x Gax 012

EU0.6 Y2.4 Fe5-x Gax 012

EU0.5 Yb0.15 Y2.35 Fe5-X G\ 012

/

0.2

/ /

o

/

0 0.9 1.0

1 1.1 1.2

X , Ion/Molecule

1.3 1.4

Figure 2. Characteristic Length vs. Gallium Content for Eu-Yb-Y Films

13

-■■■-■' 1 ■:■ j^. ■ ■;■■-. ;->,.,■.:...-:.-.■■■-j .L ■■■ .,.,.-■■■ ..J „^ ..■,. -L J":— ■■-•■■■ - - - ■ ■' ^ ■■•■ '■■ ■■■-■-' ■■ ■ warrifl>iM<llitlMI"ilftrlriYili'ii'

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Thermocouple Voltage, mV

Figure 3. Optical Hysteresigraph Tracings of Neel Temperatur« Determinations of Eu-Yb-Y Films

14

JUK.1;.^....;:.^..-...;.-.. .i-:-.-i...^.>^,.:^..^i-».^tl^.J.-.-.^.-..u A<j*c-. .v.j^^i^.^^^^^.^^^-.i.^^itij^^Am^^^^ -■■■'■■" ■-■•-■-■■■■ --■■ ■■ - ■

,— „.„„ivn,,,,,... , „.. ' - .1.1. I.. M»<^*II-*UW««JI»<"<>< "■•^■'■••"'j^w* >!■■ jiiviaiiupimiiiai i.ui q.iwa IUIMITI ix. i» < 11 iiwwnwKl ■Mil II . Jill immiimi Jin w» ■XIIIIIJ lllliup

component could be a result of the large, positive < 100> magnetostriction

coefficient of the Eu ion. If such a component is present it may affect the

operating margins of bubble devices. Therefore, it is advisable to grow

Eu-containing xilms with a close lattice match to the substrate.

3.3.2.4 . Coercivity

The coercivity of the films grown for delivery was about 0.2 - 0.3 Oe.

Such a relatively high value of coercivity appears to be characteristic of Eu-

containing films grown in tension. Lower coercivities can be obtained by

insuring a close lattice match between film and substrate.

3.3.2.5 Mobility

Mobility appears to be quite variable, both within one sample and

from sample to sample of a given growth series. This effect has been

previously reported by Vella-Coleiro'14^ and has been thoroughly verified

in the present study. While no definitive correlation with other properties

has been found, certain trends are detectable.

The response of EuY garnets containing small amounts of Yb to a

step change in bias field is exemplified in Fig. 4. The response at zero in-

plane field is characteristic of an overdamped harmonic oscillator and is

treated is discussed in reference 4. Upon application of an in-plane field,

Hjp of 40 Oe or more, heavily darapled oscillations are observed which per-

mit analysis of the type discuosed in reference 15. The frequency of these

oscillations generally increases as an in-plane field is applied while the

dec^y time exhibits a tendency to decrease, as is illustrated in Fig. 5.

15

'^(JIJ»ww^»^w-i.^*^'.-,'fj^^A^^^ ■ -^a^rswffltrvTFT^wim^riw^^wpi'^wwwrw^^

na d

.5? in

o. O

20 40

Time (nsec)

60 80

TO c

CO

(TJ o Q. o

Time (nsec)

Figure 4. Optical Response of a Typical Sample of Eu-Yb-Y Garnet to a Step Rise in Bias Field at Various In-Plane Fields

16

-- -■ i

mmmmmmmmmmmm^m-mmmifrmmmmmmmmmmmmm wmmmmmm. mmmmmmmmmmmm:s

40

30

Oi

3 20 ■o

10

0

40

• •^,

i I I I i I I I I I I

s

S

30 ISI

20

/

s

• y s

s s s

s s s s s s s*

10 l_L J I I L J I L 50 100

Hx (Oersteds)

Figure 5. Natural Frequency and Decay Time vs. In-Plane Field for a Typical Sample of Eu-Yb-Y Garnet

17

A.'fcL-iluL/.^Lfe^JuL . ...^ ^..-^-.^^ .^ -■ ^- - - — ^— ,-^..^_ ..-.^ A, . _. ti',illK-n*rfWall.-......:! .■ ..^.ti^;' ^-r.ru!

wmm*mmmmm*mmf^<mmu.mimi™mmmmmmm*miimimm^*m***^*^mim*mm^*mmmm*t mmmmmmmmmmmmmmmmmmmmm

is con- There is a reasonably linear frequency vs. Hx characteristic which u

sistently observed in all compositions in which this oscillatory behavior is

observed. The internal agreement of the Td data must be considered largely

fortuitous.

Fig. 6 illustrates the behavior of the mobility with in-plane field as

determined both from the oscillatory behavior and the over-damped results.

where these have been observed. The oscillatory result at Hx = 0 is

determined for the extrapolations shown on Fig. 5. We tentatively assign

the difference between these results to a suppression of turbulent motion of

the wall by the in-plane field. W The presence of an in-plane field in a

device will clearly improve the speed of the device.

The variation of mobility with temperature has not been a routine

measurement in this study because of the various equipment complexities

involved. It is, nonetheless, an important characteristic of any material

destined for use over a significant temperature range. We have therefore

recently initiated such measurements using the step field response method

within a thermoelectric variable temperature stage.

Mobility was found to decrease as temperature was lowered, as is

shown in Fig. 7. The wall mobility in Hx = 0 at -100C ranges from 50 to

150 cm/sec/Oe. For Hx = 33 Oe (limited by the variable temperature

apparatus) low temperature mobilities between 180 and 500 cm/sec Oe were

observed, there being a definite correlation between values for H =0 and

those forHx = 33 Oe.

18

.. , . ■.■....■.■.-J..^^i„»^J^^-.-^.-,.J—. „^.x^^x^.— L^^^^-a.^-.»^.!^-^^.!^^.^^, . _,. . ___^, -*

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a> 1500 O

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b y*

^ 1000 y

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y 500

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J i i i J L 50 100

H (Oe) x

• - Results from Oscillatory Curves and (at H = 0) an Extrapolation of those Results

0 - Results From Cverdamjjed Response Curves

Figure 6, Domain Wall Mobility vs. In-Plane Field for a Typical Sample of Eu-Yb-Y Garnet

19

U^ygjUgy,, .J„J..J..„^^:.....„:..^.:^-^^ .^.-^-^^^ .... M| - ,, , ■ " .■.■^..,-

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200

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T(0C)

Figure 7. Domain Wall Mobility vs. Temperature in a Typical Sample of Eu-Yb-Y Garnet - All Results are Over- damped Response Curves

20

i^- - - - —- -■ - ■- ' ' ' -"■-^-— - -^-

'v*^mi*wr*mwJ!fv*r*v.mm•*'wim»^v^*y9n*v*m'jrm' jmull»*™mmKm^ymmm■jflWiJ.'i«"WAWJM,i"iu'iiiwiM^nuniifMmnj(iulup,«!)^ »iwi^pl"^«*iH»^jiPHPW**,"^mw^^WEW*W«^«J'll!Wip^W"S^l*|.|l4W.w»Wlmimmmm**m&'VmmiVimmm}i*mii

There are some interesting correlations between low coercivity and

high mobility values within this system which should be further studied.

Possibly both of these desirable properties are associated with the absence

of a very small scale network of defects. Such a network, when present,

might not only hold up domain walls, thereby increasing coercivity. but

also cause the wall motion to be more turbulent and thus reduce mobility,

perhaps through nucleation of in-plane Bloch lines. (17' 18) This netWork

might arise at the film-subs träte interface from a lattice mismatch of

substrate and film too small to produce cracking.

3.4 The Sm-Y System

3.4.1 Introduction

It has been pointed out previously^2) that Sm ions are among those

which are of interest as additions to Y3Fe5_xGaxOi2. Sin3Fe50lz, like

EujFegO^, does not exhibit a compensation point. Hence, the magnetic pro-

perties of the mixed Sm-Y garnets are expected to exhibit good temperature

stability. However, the damping parameter of the Sm ion is much higher than

that of the Eu ion^ ' and, therefore, might be expected to affect the intrinsic

mobility of Y3Fe5_xGaxOj2 films to a greater extent than Eu ions. On the

other hand, since some damping is required to reduce dynamic instabilities

and to linearize the velocity-drive field relationship, mixed Sm-Y garnets

(containing small amounts of Sm) might be expected to exhibit reasonable wall

mobilities at practical drive fields. Therefore SmyY3_yFe5.xGax012 films

were among those grown and supplied to the Sponsor.

21

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3.4.2 Properties of Sm-Y Garnet Films

3.4.2.1 Saturation Magnetization

In Fig. 8, room temperature saturation magnetization is plotted as a

function of gallium content for films containing various concentration ratios

of Sm/Y. Since the films are not of the same or optimum thickness, there

is some scatter in the data. However, the genera}, trend is obvious.

Magnetization decreases with increasing Ga and does not depend strongly,

if at all, on the concentration ratio of rare-earth ions,

3.4.2.2 Claracteristic Length

The behavior of the characteristic length, I , with composition is

illustrated in Fig. 9. Again, the scatter in the data results from the films

being of different thicknesses. Characteristic length is seen to be indepen-

dent of Sm/Y concentration ratio.

Since the characteristic length of domains in films which support

stable 6|am bubbles is about 0. 75 jam, a Ga content of about 1. 3 ion/molecule

is required.

3. 4. 2. 3 Coercivity

Fig. 10 is a plot of coercivity vs Ga content of the various films.

The coercivity is quite low and independent of Ga content for x less than about

1.2 ion/molecule. For higher x, it increases rapidly with Ga content. This

behavior is believed to be related to the lattice mismatch between film and

substrate. The films are grown in tension and lattice mismatch increases

with increasing Ga. The rise in coercivity with Ga is believed to result from

23

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24

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excessive mismatch. Since a Ga content of about 1. 3 ion/molecule is required

for stable 6 Urn bubbles a good lattice match could be ensured with a Sm

content of about 0. 5 ion/molecule (based on Vegard's law).

3.4.2.4 Anisotropy

The anisotropy field and the anisotropy energy density for a number

of films are given in Table 1. These films were grown at somewhat different

temperatures and growth rates and since anisotropy is dependent on these

growth conditions, the values are not strictly comparable. However, the

values are illustrative of the magnitude of the anisotropy which can be

obtained in this garnet system. The anisotropy field of films which support

stable 6 ym bubbles is about 1500 Oe and the saturation magnetization is

about 150 Gauss. These values result in a quality factor of about 10 for

these films.

TABLE 1

Uniaxial Anisotropy of Some Sm-Y Films

Anisotropy Anisotropy

Sample No. Comp Sm

0.16 0.16

Dsition Ga

1.23 1.28

Magnetization (Gauss)

220 146

Field (Oe)

Energy Densi (erg/cmz)

5564A 5565A

970 1480

8468 17150

5567A 5571B

0.24 0.24

1.25 1.31

183 134

1220 1440

8859 7657

5577B 5578A 5586C

0.32 0.32 0.32

1.18 1.24 1.27

228 194 149

1210 1540 1500

10948 11855 8870

5562B 5562E

0.36 0.36

1.25 1.33

187 134

1770 1685

13134 8960

26

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The data of Table 1 suggest that the anisotropy energy density

decreases slightly with increasing Ga for a particular Sm/Y concentration

ratio even though the anisotropy field appears to increase. This is probably

a consequence of the rapid decrease of magnetization with Ga content (Fig. 8).

3.4.2.5 Mobility

Sm-Y films, when subjected to a step increase of bias field, always

exhibit a simple wall motion to the final position (usually exponential) rather

than an oscillatory motion. This is in keeping with the higher damping to be

expected from Sm ions compared with Eu or Yb. There are additional

problems present, however, which are presumably associated with the pre-

sence of Bloch lines in the domain walls. A typical sample which has been

subjected to a variety of previous treatments will exhibit a characteristic

response time (to 63% of final signal) of ~ 200 nsec. at Hx = 0. During this

measurement the stripe domain pattern is observed to slowly move and

change form. A similar slow response is often observed at Hx = 40 Oe.

However, when the sample is subjected to a large in-plane field or a

saturating bias field and then returned to Hx = 40 (not going through zero) a

response approximately an order of magnitude faster is observed. In this

condition the stripe pattern does not move visually. A reasonable explanation

is that Bloch lines, originally in the walls, have been disposed of and the

latter response is charac'.eristic of walls free of Bloch lines. If Hx is now

returned to zero one can often observe the pattern begin to move, first in an

edge of the field of view and gradually spreading throughout as Bloch lines

27

-■ ■■ ..„ J. u ^.^.„„.„^.^„„.^^.^^ ^ .^ -■■• ^^Ä«^«fii«»^siiua|j||^^

^^n^*?*^WfWns?WPp^T?rTWW*r^^ IWTI ^U^^W^'»"^«^»^"^.^ •?^^»fl^flm5BW?^5W^W7f7?n^W?^^:W^^

spread through the pattern. One would expect that during this process a

combination of the fast and slow responses might be seen. This has indeed

been observed. In what follows emphasis is given to the faster response as

that to be expected in a film in which hard bubbles have been suppressed

(for example, by ion implantation).

The mobility values versus in-plane field are given for three compo-

sitions in Fig. 11. From the foregoing discussion it is clear that the

values near Hx = 0 may be depressed by the presence of Bloch lines. How-

ever by Hx = 40 Oe this complication should be absent and, indeed, the data

show the qualitative reduction of mobility to be expected as more of the

highly damping Sm ion is introduced. Variability within a composition

remains a problem with other samples of the x = 0. 32 composition showing

mobility at Hx = 40 as low as 180 cm/sec/Oe. It is clear, however, that

quite useful mobilities are possible in this composition.

3.4.2.6 Temperature Coefficient of Characteristic Length

The characteristic length was measured as a function of temperature

from about 0 to 50oC and the average temperature coefficient of characteris-

tic length (100^ £/£ AT) was calculated for that temperature range. The

coefficient was about 0. 5 percent/degree for all the compositions studied.

3.5 Nd-Y System

Epitaxial films of NdyY3_yFe5_xGax012 were deposited on (ill)

Gd3Ga5012 substrates. The nominal value of Nd ranged from 0.4 to 0.67 and

x ranged from 1. 0 to 1.2. The films were smooth and were not cracked.

28

am,;-:.:.^.^^- .... -^.. - ■.,^.J-.,..„.;-.,J^-^...^,.....-. „.J.J,^a..^..^^^>...,.....„.,J^„^^........,^.^,....„ni„.^..v..^.-. ■ .,..,.-.„ ...„. >.. ^.. „ _ . _.„_ ■^...„:^....,.,..„ ..,

. in HU ■iimi.ji in ..,^w-.....«i.i.u«~—T, j iLi,iiiuia..iini .H i niii > jaannn.Baai u! i. n IIKüI MPIUMI 11 11 u . » ji. i •. > 11 jwiii uanpHnnnininmn«^^^«'

10C0

o

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800

600

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200

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I 50 100

In - Plane Field, Oe

150 200

Figure 11. Mobility vs. In-Plane Field for Some Sm,rYq „Fe, GaxO, „ Garnets

1yI3-yr e5-x

29

.^^a^,^...^^:-.-.■■...— ^^^...■^■^^.■i.^ ^^.i^J...^<V^*^*i)llt]iföa±i^ ■-■lVt-^..^i.^W^.^,J^^^..,.^,..;^,.J... .... ;„., .■..,-„■..■......- „;■..■ ..-..■.:■■:.,:... A.^-^^^.^:,.^.^...

Thicknesses ranged from about 3 to 20|-im. The films did not exhibit uniaxial

anisotropy in the < 111 > direction, the magnetization apparently lying in the

plane in all c?,ses. Anamolous anisotropy effects have also been noted in

bulk, flux-grown, Nd-containing garnets by Gyorgy, etal.v1^'

3.6 Discussion

Both the Eu-Yb-Y and Sm-Y systems appear to be promising candi-

dates for bubble device application. The garnets exhibit reasonable

mobilities, satisfactory quality factors and good temperature stability.

Coercivity can be minimized by assuring a close lattice match between film

and substrate. Compositions which support stable 5 - 7 Mm bubbles have

been identified.

Additional work with these systems could be quite rewarding. The

relationship between mobility and composition is not entirely clear. In

addition the proposed relationship between mobility and coercivity particu-

larly apparent in the Eu-Yb-Y system deserves further study. Such a rela-

tionship could have an important bearing on bubble technology regardless of

film composition. A detailed study might lead to a better understanding of

wall mobility and wall turbulence in garnet films.

30

^atemm uut^^mummmtmmmmmtmmi lüHHlMMMHMMIk ■ ■

4.0 PLASMA SPRAYING

4. 1 Introduction

Plasma spraying was investigated to determine its potential to pro-

duce magnetic garnet films of the composition GdEr2Fe4.5Ga0.5O12 with

superior or unique properties, or to produce magnetic films with satisfactory

characteristics at lower cost than traditional crystal growth processes.

Two approaches were designated for study: (1) the conversion of

plasma-sprayed polycrystalline garnet films into monocrystalline films,

and (2) the direct fabrication of plasma-sprayed monocrystalline garnet films

The first approach was a two-step process for forming monocrystalline

films. It consisted of spraying polycrystalline films containing garnet and

flux and subsequently converting the deposits into monocrystalline garnet

films by a thermal treatment. Results of this work were reported in the

Semi-Annual Technical Report. (2)

The second approach was a simplified process in which the film

deposition and thermal treatment were performed in close order to reduce

the operation to an essentially single step. This method was studied during

the second portion of this contract and results of this work are reported

in this section of the report.

4. 1. 1 Growth from the Melt

The first and most straight-forward approach at garnet film growth

by direct deposition was growth from a melt. Garnet powder was fed

slowly into the D. C. arc plasma torch and deposited as a molten film. This

approach was unsuccessful due, primarily, to the temperature variations

31 Kfai.-..^^.^..,..,...,...

■ ■ ■ ■■ ■ ■ ■■■

experienced by the substrate during plasma spraying and the inability to

maintain the stoichiometric compoeition of the incongruently melting garnet.

Films deposited in this manner were essentially polycrystalline.

4,1.Z Growth from Solution

Other studies were made using a modified liquid epitaxy approach

similar to the work performed during the first period of this contract, except that

both plasma spray deposition and subsequent single crystal film growth

were accomplished in the same apparatus without interim cooling to room

temperature. The heated particulate was sprayed onto the substrate and

formed a molten solution of flux and garnet on the subst^te surface. Film

growth was initiated by cooling the solution to a preselected growth tempera-

ture.

After several conversion runs were made using the D. C plasma

torch as both the means of powder deposition and the substrate heat source,

an auxiliary resistance furnace was found to be a more stable and uniform

high temperature heat source for film growth. Thereafter the D. C. plasma

torch was used only as a means of providing heat to the powder for melting

and deposition on the substrate.

The D. C. plasma heat source was abandoned in favor of the R, F.

plasma source early in the program because of the latter's lower gas

velocity and resulting gentler deposition of the molten particles. In

addition, the absence of consumable electrodes in the R.F. plasma resulted

in a cleaner film growth environment. As with the D. C plasma, the use of

an auxiliary heater during film growth provided a stable and uniform growth

temperature.

32

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The best bubble garnet films prepared during the second period of this program

were grown using the R.F. plasma torch in conjunction with an auxiliary heater.

4. 2 Experimental

4.2.1 R.F. Plasma Spraying

The radio-frequency (R.F. ) plasma process used during most of this

study was first proposed by T. B. Reed(20). It represents an attractive way

to heat a particulate to a high temper iture without introducing electrode

impurities. The overall process and equipment are shown schematically

in Fig. 12.

Electrical power is supplied to a load coil by means of a 30-kW

induction heating unit (Lepel, Model T-20-3-DF 1-E-H) having a resonance

frequency of ~ 3. 5 magacycles. The plasma acts as a 1-turn secondary coil

to the 4-turn load coil. A shielded coaxial copper tube is used on the positive

side of the R.F. coil to prevent energy loss.

The R.F. plasma is initiated by a high-voltage discharge produced

with a Tesla coil. Once the argon gas is partially ionized, it couples

efficiently with the R.F. field and sustains itself continuously as long as

plasma-forming gas is supplied. In order to efficiently initiate a plasma, the

impedance of the power unit and of the load coil must be properly matched.

By so doing, the reflected power is substantially reduced.

The R.F. plasma torch (shown in detail in Fig. 13) consists of two con-

centric quartz tubes with three independent gas streams. Cooling air flows

between the tubes to prevent melting. It is blocked from entering the hot

zone by a graphite ring. Argon gas flows through the inner tube and forms

33

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-- ■■ ■ ■■ "- ■HM» - - - - - -—- —...._. . ■ .. ^

Feed Powder Inlet

J

'O'-Ring

Inner Quartz Tube

[I j Water-Cooled Powder Feed Tube

Rubber Ring

Plasma-Forming Gas Inlet

Outer Quartz Tube

Plasma Flame

Substrate

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Heating Furnace

Alumina Pedestal

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R.F. Coils

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Alumina Tube

nsulation

Furnace Windings

Specimen Thermocouple

S.S. Tube

Thermocouple Wire to Recorder

Figure 13. R. F. Plasma Torch Detail

35

r^»..^.JA,,^-.-...mV.,^..^^^--..^u-^.^..^.,^^.^...^J.a,^M»».^^.^>.JJi^^.^~M-^. IM :-:-^--.^. ■■ .... , . ,. ,.^ •^.«.«„ö^m

the main body of the plasma. It is injected tangentially so as to create a

vortex which forms a low pressure region in the center of the plasma.

Circulation of some of the ionized gas up the center of the plasma provides

enough seed gas to continuously maintain the plasma. Finally, oxygen gasi

flowing through a water-cooled copper tube, carries powder through the high-

temperature plasma for melting and deposition on the pre-heated substrate.

A cylindrical 4-turn R.F. coil of copper tubing surrounds the outer

tube and serves to couple energy into the plasma. To provide a uniform

preheat and growth temperature over the substrate, an auxiliary furnace is

fitted around the outer quartz tube below the induction coil.

After preheating the GGG substrate to a specific temperature in the

auxiliary furnace, the substrate is quickly removed from the furnace and

positioned at the appropriate distance from the powder tube outlet. Powder

is transported from a feeder using an oxygen carrier gas and passed through

the high-temperature region for heating in the plasma stream. Powder is

thus melted in the plasma flame and deposited on the substrate. The sub-

strate temperature is monitored continuously using a Pt, Pt - 13% Rh

reference-grade thermocouple which is in physical contact with the bottom

of the platinum substrate holder. After deposition, the sprayed substrate is

quickly returned to the furnace for film growth and held for a specific time.

Once film growth is terminated, the flux is decanted from the surface and

residual flux is then removed using a dilute solution of HNO3.

36

I ■ -—-■ ^-^~^w^^-—-. ^^—^....U^^ , . ...._. „„., ,,... ., n,.^

4. 2. 2 D. C. Plasma Spraying Station

Initial experiments to fabricate bubble garnet films by spraying

without cooling to room temperature and subsequently reheating to form the

epitaxial film were made using the D C. arc plasma torch discussed in the

Semi-Annual report.^)

The D.C. plasma spraying station is shown in Fig. 14. The plasma

torch is mounted against a metal bellows which allows movement of the spray

torch to permit uniform coverage while minimizing contamination from cir-

culating air during the powder deposition. A quartz tube is placed against

the bottom of the bellows to act as an enclosure for the substrate during

spray deposition and film growth. The auxiliary furnace is placed around

the quartz tube to provide a uniform growth temperature across the substrate.

Once the substrate has reached the preheat temperature, the plasma torch is

activated and powder is injected through the plasma. The molten particulate

then collects on the substrate and the specimen is held at the growth tempera-

ture for a specific time as required to form an epitaxial garnet film. After

growth, excess flux present is removed in a dilute solution of HNO3.

4. 2. 3 Material

Gadolinium oxide (Gd203), erbium oxide (E^C^), and boron oxide

(B2O3) were obtained from Research Inorganic/Organic Corporation and were

99.9%, 99.9%, and 99.99% pure, respectively. Grade I iron oxide (Fe^),

gallium oxide (Ga203) and lead oxide (PbO) manufactured by Johnson

Matthay Chemicals Limited, were obtained from United Mineral and

Chemical Corporation,each having a purity of 99.99%.

37

■ ■ ■

---— - ■ 1 '•--—■—^—^■^^^^■;...„,.-^..„^.^^.^.M^I^^I—^-^..■.^..^^^,^—L^^.^.^^^U.^^. ,.,. ..,...-.■....— _ „__,.... ... _ ....... ■—- --^- 11 rii

Plasma Flame

Substrate

Platinum Holder

--Metal Bellows

Heating Furnace-

Alumina Pedestal

—Plasma-Forming Gas Inlet

— Feed Powder Inlet

Quartz Tube

Alumina Tube

Insulation

Furnace Windings

Specimen Thermocouple

— Thermocouple Wi re to Recorder

Figure 14. D. C. Plasma Spraying Station

38

-■■■'■■.■■■ ■■.■..'■■.. ■...-... .....,-,.■ ....,.J. :-^;-^^- ^^^-^^.^ --.■.^.....-..■,- .; J..^,^u..^-.^^.:^..^:^-..^Y^IlMWlVü""^"^*-^^^■"^■■■-""-^ ^■.-■^■^-■■-J-^W-.-M^.--M. ■..>■ ■ . ..- .....- ...... . ■....■..■.■■■.. .. ■■.■ . , : ,..■■ ;.. ; .v.-... ^ ■ ......1 .. , A ,-....■■■. . ■ ■ ■ - ■ ■■■■■■-

The individual oxides were weighed to four significant figures using

an Ainsworth triple beam balance. The oxides were combined to form the

mixture shown in Table 2 and then blended for one hour.

The blended oxides were placed in a platinum beat and heated for

Z hours at 1300oC in air. The sintered charge was then pulverized into a

fine powder which after sifting, was suitable for injection into the plasma

torch and used in studies to determine the feasibility of producing epitaxial

garnet films directly from the melt.

A homogeneous mixture of the oxides in the garnet and flux powder was

obtained by forming a solution at high temperature. The solution was heated

in a closed platinum crucible at ~ 1100oC for 2 hours and then quickly poured

into a platinum boat. The resultant ingot was pulverized using a mill

containing a tungsten carbide rotating blade to form a powder which, after

sifting, was suitable for injection into the plasma spray torch for producing

films from solutions..

The < 111 > GGG substrates (~ 14 mils thick) were all obtained with

one side polished. Some substrates ^3/4" diameter) were scribed on the

unpolished side and broken into quarters in order to conserve substrate

material. Other (~ 1/2" diameter) substrates were used as received. The

substrate cleaning procedure prior to spraying has been described

previously. '^/

4. 3 Results and Discussion

As in the first portion of this contract, the approximate composition

GdEr2Fe4#5Ga0>5O12 was studied. This garnet exhibits growth-induced

39

TABUE 2

Powder Compositions

Garn< it Pc for

)wder

Growth f rom the Melt Component Weight %

Gd203 4.49

FejjOj 77.66

Er203 13.05

Ga203 4.80

Garnet & Flux Powder for

Growth f rom Solution Component Weight%

PbO 91.26

B203 1.83

Gd203 0.31

Fe203 5.37

Er203 0.90

Ga203 0.33

40

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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^mf^^^^^^^^^mrn^^^^^^tmmmm^mmmm^mr^msmm

anisotropy perpendicular to the film plane. A good lattice constant match

is obtained between this garnet and < 111 > GGG substrate.

4. 3. 1 D. C. Plasma Spraying

4. 3. 1. 1 Epitaxial Crystal Growth from the Melt

Efforts to form single crystal garnet films directly from the melt

were largely unsuccessful. Powder sprayed using the D.C. plasma torch

was slowly deposited on GGG substrates, maintained at 1200 - ISOCC, and

then slowly cooled to room temperature. However, the resultant deposits

were essentially polycrystalline. This was attributed to temperature non-

uniformity and temperature fluctuation within the substrate during spraying.

4. 3. 1. 2 Epitaxial Crystal Growth from Solution

Magnetic gr rnet film growth was achieved by using the D. C. plasma

torch for depositing a film of garnet and flux on a preheated substrate and

subsequently maintaining the specimen at the film growth temperature for

several minutes. However, domains were observed only in isolated areas

of the film after conversion and the results were not reproducible. Typical

conditions used for producing a garnet film by D. C. arc plasma spraying

are shown in Table 3.

In general, the films produced by D. C. arc plasma spraying were

inferior to those produced by the R.F. plasma spraying method. The high

kinetic energy imparted to the injected particulate caused the molten flux

and garnet to flow off the substrate during deposition. Consequently, only

a portion of the sprayed solution was available for growing an epitaxial

film on the GGG substrate.

41

- -~*^^~.^—^. iaMim. . i i— --"■ -- : " *•■ <Si

•*~™rTTr!?*r*>*f^.**w*rv?*^^

TABLE 3

Typical Conditions for Growing Epitaxial Garnet Films from a Flux

Using D. C. Arc Plasma Spraying

Spray Torch

Arc Gas

Arc Current

Plasmadyne Model SG-3 (25 kW)

Argon at 50 cfh

250 amps

Powder Feed Mechanism

Type

Carrier Gas

Plasmadyne Rotofeeder

Oxygen at 15 cfh

Growth Conditions

Substrate Preheat Temp.

Growth Temperature

Growth Time

930oC

930oC

10 minutes

42

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i. 3. 2 R.F. Plasma Spraying

The R.F. plasma method of growing bulk single crystal oxides was

succesifully demonstrated by Reed. (21) However, the oxides in his work

were simple congruent melting compounds in which bulk crystals were

grown directly from the melt. The difficulty in maintaining the correct

composition of the bubble garnet from the melt made growth from a solution

more attractive. However, garnet films made from a solution were

inferior in overall quality as compared to films made by liquid epitaxy using

the "dip process. "

The R.. F. plasma method has several drawbacks when compared to

the liquid phase epitaxy method. First, the temperature uniformity across

the substrate obtainable by the "dip process" is difficult to achieve using

an R.F. plasma. Temperatures vary from the center of the plasma to the

boundary causing composition and film thickness variations in films grown

by R. F plasma.

High defect density can probably also be attributed to the plasma

process. This conclusion is based on a liquid epitaxy "dip" experiment

using garnet-flux prepared by melting the powder employed in R.F. plasma

growth studies. In addition, GGG substrates were prepared in the manner

typically used for spraying studies. The time-temperature conditions pre-

viously found satisfactory for film growth by the "dip" process were used.

Nearly defect-free films were grown indicating that substrate and powder

preparation procedures were satisfactory. Apparently, the exposed surface

of the GGG substrate during and before plasma spraying was vulnerable to

43

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airborne contaminants. Also, the long exposure time of the substrate to the

flux probably contributed to the high defect density.

Finally, it was difficult to maintain the sprayed solutions in the

garnet stability range. The films were invariably contaminated with

crystallites of orthoferrite, hematite or other phases.

4. 3. 3 Characteristics of Selected Garnet Films

All films produced in this program were qualitatively examined using

an optical microscope to determine defect density and overall film quality.

Domain patterns were observed using polarized light. Film color was

macroscopically noted.

None of the garnet films made by D. C. plasma spraying from the

melt exhibited magnetic domains. However, films primarily sprayed from

solution did display domains over small isolated areas.

During the second phase of this program, 47 garnet films were

deposited on GGG substrates using the R.F. plasma spraying method. In

general, domains were observed in most of the films. Defect density was

usually high and the surface of such films was uneven or faceted.

Five selected garnet films representative of the direct fabrication

method using the R.F. plasma were further characterized and the results

are summarized in Table 4. The spray process conditions used in growing

these epitaxial films from the flux are shown in Table 5. In all samples,

the variation in film thickness and the high defect density made magnetic

characterization diffirult using conventional techniques.

Sample films representative of the results obtained by the spraying

process were delivered to the Sponsor.

44

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5. 0 BULK CRYSTAL GROWTH

At the beginning of this program Gd3Ga5012 substrates of 15-18 mm

diameter were available commercially. Although the dislocation and inclu-

sion densities were acceptable the crystals contained an undesirable, highly

strained central core. The lattice parameter of the core was found to be o

0.001 - 0.002A larger than the surrounding region. The Czochralski growth

of Gd3Ga5012 developed rapidly during this program and now nearly perfect,

core-free crystals of up to 38 mm in diameter are available. Lattice para-

meter variations within the crystal of less than 0. 0005Ä can be maintained.

There appears to be no inherent reason why even larger crystals of the

same quality cannot be grown.

The conditions required to grow high quality, core-free gallium

garnet crystals have been adequately discussed in previous reports^1' 2)

and will not be detailed here. The optimum rotation and growth rates for

crystal growh were found to depend on the amount of furnace insulation,

thermal gradients within the furnace, and the relative diameter of the

crystal and crucible. Hence, the "optimum-conditions are peculiar to a

particular furnace arrangement and it is not possible to specify rotation

and pull, rates that can be used in general. These conditions must be

determined empirically for each furnace arrangement.

During the second period of this contract, high quality single crystal

of Gd3Ga5Oi2 were grown routinely to supply substrates for epitaxial films.

47

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iM-iiMKini-i-i-ui« M - in im<mmrmm'mi^immmmmm>i->">>'mi,< „ imtwm*mmmKm^mmmr*mmmmm*mmmmmmmmmmmmmim*'*''*H>mim

6. 0 CONCLUSIONS AND RECOMMENDATIONS

The technology of garnet bubble materials has advanced rapidly during

the last two years. This is due in great part to the development of LPE

growth of garnet films. Levinstein, et al. ,(11) discovered the remarkable

stability of supercooled solutions of garnet in PbO-B203 and first described

the simple "dipping" method of film growth. Since then, the technique has

been used extensively in the preparation of multicomponent, uniaxial garnet

films. This is the method used to prepare the high quality films supplied

the Sponsor during this project.

Most of the garnet LPE work has been done in PbO-based solvents.

However, Linares*22^ investigated other solvents including those based on

BaO. Recently, the BaO-based solvent has been the subject of an intensive

investigation by workers at Hewlett-Packard Laboratories. (23) This solvent

was also investigated in this work. (!) The BaO-based solvent exhibits

several desirable features including low volatility, n&n-corrosiveness and

low toxicity. However, the high viscosity of the melt at practical film

growth temperatures and the attendant difficulties of solvent drainage from

the film have been serious deterrents to its widespread use.

Epitaxial garnet films have been prepared by methods other than LPE,

including CVD, APS, sputtering and coprecipitation/annealing. The CVD and

APS processes have been studied as a part of this project. High quality films

can be prepared by CVD but it is extremely difficult to achieve run-to-run

reproducibility with the complex garnet films of interest to present day bubble

technology. Most workers in the field have now abandoned the CVD approach.

48

IM_ '^i^**^*^^****^^

■ iiii",pia ("n ■ .,.,-...1,..*- wmmimmmmim^mm™ <n'*'i> >'"'"• ~*~*~~^mimmBmmmmm*mammmmmmmmmw,mw*'mimm

In respect to APS. the major problem appears to be control of phase rela-

tionships. Epitaxial garnet films grown by APS are usually contaminated

with crystallites of orthoferrite. hematite, magnetoplumbite and/or other

solid phases. Low defect density films have not been grown and the

elimination of unwanted phases appears to be a formidable problem.

An understanding of the mechanics and kinetics of LPE film growth

has been developed^4' 25) and the reproducibility of the process has been

demonstrated. (26) Aii recent developments indicate that LPE "dipping"

growth of garnet films is a commercially viable process. Remarkable

progress has been made in the relatively short time the process has been

used to grow magnetic garnet films. It is reasonable to expect that

additional progress will be made in the future. It is believed that nearly

all workers in this field are now using the LPE "dipping" method and this

situation is expected to continue.

The growth of bulk, non-magnetic garnet crystals for substrates has

kept pace with the development of epitaxial film growth. Core-free single

crystals with dislocation densities less than 5 per cm^ can be readily grown

by the Czochralski technique. Such crystals, up to 38 mm in diameter, are

now available from commercial vendors.

A number of film compositions were delivered to the Sponsor during

this project. These films, tailored to support 5 - 7 ^im bubbles, represented

a variety of bubble materials. Included were films of primarily stress-

induced anisotropy, primarily growth induced anisotropy and films in which

49

b'"-^-" - — —^..T""

" ■ i" i i i " ■ i ■!■ 'I ■' i mmr^^mKimmtintmMaiiw JII 11 lima*|l«MIW|HPIW<mw«aM«*^^*V«^n«WWn«HOTMI^MVnnii^^^^^mn>.

both growth and stress-induced mechanisms were important. At least two

garnet systems. Eu-Yb-Y. and Sm-Y were identified as being of particular

practical interest. These garnets exhibit reasonable mobilities, satisfactory

quality factors and good temperature stability.

The "hard" bubble problem was not recognized when this project

was initiated and has not been a subject of this work. However, two

different, more or less auccossful solutions have been developed by others.

These are (1) the epitaxial deposition of a "capping" layer^27) and (2) ion-

implantation. (28)

Although a good understanding of garnet bubble films has been

obtained to date, additional work is needed. In particular, it is felt that

detailed study of the possible relationship between wall mobility and

turbulence and coercivity might be especially rewarding.

50

^»^^■..^.w:-.....:va.^^^^U^

..M.. "".M.". J...-..1I!. .i> \nwimmm*mi^*m****^mmmm^^mmm^memm~i~~m'~^**~~*'^^mmmmmmmiHmm*^^^mf

7 . 0 REFERENCES

1. J. W Moody. R. M. Sandfort, and R. W. Shaw, Final Report. Magnetic Bubble Materials, MRC-SL-341, Contract No. DAAH01-72-(>0490~ August 11, 1972

2. J. W. Moody, R. M. Sandfort, R. W. Shaw, and R. J. Janoweicki. Semi-Annual Technical Report. Magnetic Bubble Materials MRC-SL- 354, Contract No. DAAH01-72-C-1098, Feb. 11, 1973'.

3. R. W. Shaw, R. M. Sandfort, and J. W. Moody, Magnetic Bubble Materials: Initial Characterization Report. MRC-SL-326, Contract No. DAAH01-72-C-0490. March. 1972

4. R. W. Shaw. R. M. Sandfort; and J. W. Moody. Magnetic Bubble Material; Characterization Techniques Study Report. MRC-SL-339, Contract No— DAAH01-72-C-0490, July, 1972

5. D. C. Fowlisand J. A. Copeiand. "Rapid Method for Determining the Magnetization and Intrinsic Length of Magnetic Bubble Domain Materials", AIP Conference Proceedings. No. 5. 240 (1971).

6. R. W. Shaw. D. E. Hill. R. M. Sandfort, and J. W. Moody, "Determina- tion of Magnetic Bubble Film Parameters from Strip Domain Measure- ments". J. Appl. Phys.. 44, 2346 (1973).

7. A. A. Thiele, "Theory of the Static Stability of Cylindrical Domains in Umaxial Platelets", J. Appl. Phys., 41., 1139, (1970).

8. P. W. Shumate, Jr., D. H. Smith and B. F. Hagedorn, "The Tempera- ture Dependence of the Anisotropy and Coercivity in Epitaxial Films of Mixed Rare-Earth Iron Garnets", J. Appl. Phys.. 44. 449 (1973).

9. P. W. Shumate, Jr. , "Extension of the Analysis ^or an Optical Mangeto- meter to Include Cubic Anisotropy in Detail", J. Appl. Phys., 44, 3323 (1973). —

10. R. M. Josephs. "Characterization of the Magnetic Behavior of Bubble Domains". AIP Conference Proceedings. No. 10, 286 (1973).

11 H. J. Levxnstein, S. Licht, R. w. Landorf, and S. L. Blank, "Growth of High Quality Garnet Thin Films from Supercooled Melts" Aool Phys. Letters, 19_. 486 (1971). ' PH •

51

^^■v.^....,.....^.^.^^^ -,1,.,.,.,J...„:J..,..^.^,.,J.1...,.,.U........,.^-..:- -.■■. OM

'^»^»^VPWWlffffWIWIRWW'^S^tiPW'^SW^K^^

12. W. A. Bonner, J. E. Geusic, D. H. Smith, F. C. Rossol, L. G. Van Uitert, and G. P. Vella-Coleiro, "Characteristics of Temperature- Stable Eu-Based Garnet Films for Magnetic Bubble Application", J. Appl. Phys. 43^, 3226 (1972).

13. NASA Contract No. NAS1-11794

14. G. P. Vella-Coleiro, "Domain Wall Mobility in Epitaxial Garnet Films", AIP Conference Proceedings, No. 10, 425 (1973).

15. Jerry W. Moody, Roger W. Shaw, and Robert M. Sandfort, and R. L. Stermer, "Properties of GdyYs-yFeg-xGaxOjz Films Grown by Liquid Phase Epitaxy", Intermag 1973, Proc. , to be published.

16. B. E. Argyle and A. P. Malozemoff, "Experimental Study of Domain Wall Response to Sinusoidal and Pulsed Fields", AIP Conference Proceedings No. 5, 115 (1972).

17. J. C. Slonczewski, "Theory of Domain Wall Motion in Mangetic Films and Platelets, " J. Appl. Phys. 44, 1759 (1973).

18. E. Schloemann, "Light and Heavy Domain Walls in Bubble Films", AIP Conference Proceedings, No. 10, 478 (1973).

19. E. M. Gyorgy, M. D. Sturge, L. G. Van Uitert, E. J. Heil-Aer, and W. H. Grodkiewicz, "Growth Induced Anisotropy of Some Mixed Rare-earth Iron Garnets", J. Appl. Phys. 44, 438 (1973).

20. T. B. Reed, "Induction-Coupled Plasma Torch", J. Appl. Phys., 32. 821 (1961).

21. T. B. Reed, "Growth of Refractory Crystals Using the Induction Plasma Torch", J. Appl. Phys. 32 , 2534 (1961).

22. R. C. Linares, "Epitaxial Growth of Narrow Linewidth Yttrium Iron Garnet Films", J, Cryst. Growth, 34, 443 (1968).

23. see, for example, R. Hiskes and R. A. Burmeister, "Properties of Rare-Earth Iron Garnets Grown in BaO-Based and PbO-Based Solvents", AIP Conference Proceedings, No. 10, 304 (1973).

24. R. Ghez, and E. A. Giess, "Liquid Phase Epitaxial Growth Kinetics of Magnetic Garnet Films Grown by Isothermal Dipping with Axial Rotation", Mat. Res. Bull, 8, 31 (1973).

52

^.^..:,:..:, ......-■ "-.-^ ...■ ■ :....;.■ .-.^--...-..-.--,.. ^■u,-.!...-!.^-. .■,.■■■■... dsä^tt - -,...-..■.. ■■.....,. -■.,--^ ,.__^üÄ

■■-"■^fl.^ip.u^^^iiiV«'«,"ä»W^ i»?.i^.-^.."ir»'»v-«WTWL-■'■J"."Jj",!!»" *' i" .'l,*^M*,■^f■!^lW''-l-.■1^l■-■ w^^^ww^pwrw^^TFVT-Tjrsw^r^eww?^^^ ^ft»wj#-Jirr« i ■.^...jr,wi,1.wpji,-1M"pijv»ltiw'i|.«ii.) n JIWI^IJII uimi,. ^iMuipu^«iiJ!Ji..(iJiil.ium^sniqPi

25. S. L Blank, B. S. Hewitt. L. K. Shick, and J. W. Nielsen, "Kinetics of LPE Growth and Its Influence on Magnetic Properties", AIP Conference Proceedings, No. 10, 256 (1973).

26. B. S. Hewitt, R. D. Pierce, S. L. Blank and S. Knight. "Technique for Controlling the Properties of Magnetic Garnet Films", Intermag Conference, Washington D.C., 1973. to be published.

27. A. H. Bobeck. S. L. Blank, and H. J. Levinstein, "Multilayer Epitaxial Garnet Films for Magnetic Bubble Devices - Hard Bubble Suppression' Bell System Tech. J., 51, 1431 (1972).

28. R. Wolfe and J. C. North, "Suppression of Hard Bubbles in Magnetic Garnet Films by Ion Implantation", Bell System Tech. J.. 51. 1436(1972). —

53

■■-.■--'- «■-- ~- -' ■■-.■ ■■-■.■ ...■....^. ....-.,<—.. .„.. -.-■....,■.„-■.,„,..,.,

mmiii!r**r!rmzv?**Bm**y*W*W . *!!T^.r^*fl^-'l^l'^WWTO»v:w..i»V^

8.0 APPENDIX

8. 1 Growth Parameters for Eu0.sYbaa5Y2.35Fe3.8Ga1.2O12

Solution Composition:

Compound

PbO B203

Eu203 Yb203 Y203 Fe203

Ga203

TOTAL

Liquidus Temperature: Growth Temperature: Rotation Rate:

Wt

400.0 8.000 0.7374 0.2477 2.2237

25.2933 4.0491

440.6

976 C 970 C 200 rprn

Moles

1.792 0.1149 0.002095 0.0006285 0.009846 0.1584 0.02160

2.099

Mol %

85.37 5.474 0. 09980 0.02994 0.4691 7.546 1.029

100.0

8. 2 Growth Parameters for Sm0.36Y2.64Fe3.7Ga1.3O12

Solution Composition:

Compound

PbO B203 Sm203

Y203

Fe203

Ga203

TOTAL

Liquidus Temperature: Growth Temperature: Rotation Rate:

Wt

400.0 8.000 0.6293 2.6482

22.8199 4.0030

438. 1

977 C 970 C 2.00 rpm

Moles

1.7920 0.1149 0.001804 0.01173 0.1429 0.02136

2.085

Mol %

85.95 5.511 0.08652 0.5626 6.854 1.024

100.0

54

,-v--^......^ ■■■ - - ■- ■■"- ■ ■. ■ ....■.—.- ..-.,..... ,....^...,.. ,....„...^. ,,, ,„.., , -■■-■■.-:■. !•• IIH ■'r'i<l^■»■lW'-t::*a^*